1. Field of The Invention
The invention relates to an improved process for preparing ketones, and more particularly relates to an improved process for preparing pseudoionones by the condensation of citral and a ketone.
2. Brief Description of The Prior Art
The prior art is replete with descriptions of methods and procedures for preparing pseudoionones by the condensation of citral with a ketone. Exemplary of such descriptions are those pertaining to the condensation of citral with acetone to prepare pseudoionone.
Pseudoionone, also known as 6,10-dimethyl-3,5,9-undeca- trien-2-one, is an unsaturated ketone useful as an intermediate in the synthesis of vitamin A and odorants (such as ionone). A number of synthetic routes for its preparation have been developed. Some of these routes, which give a less pure product, are based on treatment of oil of lemon grass and acetone with bleaching powder, cobalt nitrate, and alcohol [Ziegler, J. Prakt. Chem. [2] 57, 493 (1898); Tiemann, Ber. 31, 2313 (1898); Haarman, Riemer & Co., German Patent No. 73,089].
The route which has been used in the prior art to make relatively pure product is based on condensation of citral with acetone in the presence of a condensation catalyst. The condensation catalysts which have been used have been solutions of bases in solvents, usually in water or alcohol. A number of these prior art processes are summarized in Organic Syntheses, Collective volume 3, 750 and utilize as bases aqueous barium hydroxide, a solution of sodium ethoxide in ethanol, metallic sodium (which is predissolved in alcohol according to the primary reference), and alcoholic sodium hydroxide. U.S. Pat. No. 3,480,677 to Meuly et al. (Nov. 25, 1969) cites the use of aqueous sodium hydroxide in the condensation of citral with acetone, and U.S. Pat. No. 3,840,601 to Gradeff (Oct. 8, 1974) cites the use of solutions of sodium hydroxide or sodium alkoxide in the condensation of citral with a ketone. The use of an aqueous sodium hydroxide solution as the catalyst for the citral-acetone condensation reaction has been commercially practiced. These prior art processes provide relatively impure crude products which must be purified by a painstaking and costly washing operation followed by a fractional distillation which must be done very carefully to remove close boiling by-products. Moreover, this requirement for distillation exposes the product to prolonged heating with formation of more by-products. The decomposition during distillation is aggravated by the presence of either the alkaline catalyst or any acid used to remove the alkaline catalyst. Moreover, the processes of the prior art generally have given satisfactory yields only with the use of a large excess of acetone, typically 10-20 moles per mole of citral, thus cutting down the space yield of the process and requiring a large and costly plant for a commercial scale of production.
The method of the present invention makes available a process for the preparation of pseudoionones with improved yields, at a rapid rate, and with simplified process operations. The product obtained is of a relatively high purity.
The method of the present invention also has a number of other important advantages. Namely, it simplifies a following distillation requirement since the crude product is of high assay with less foreshot and less high boiling residues to be removed. Lower quantities of isocitral aldols are produced as by-products with consequent fewer high boiling by-products to be separated. It also permits a simple filtration removal of the unused catalyst, allowing at least part of it to be recycled. Also, it allows the unused excess ketone to be recovered in a substantially anhydrous state (when the process is carried out under anhydrous conditions) facilitating recycle. The method of the invention is readily adaptable to continuous operation, using a packed column of catalyst, or a stirred vessel operated in continuous mode.